Tool cutting into gold for so long (5)
I believe the answer is:
adieu
'for so long' is the definition.
(both can mean goodbye)
'tool cutting into gold' is the wordplay.
'tool' becomes 'die' (die is a kind of tool).
'cutting into' means one lot of letters goes inside another.
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'die' going within 'au' is 'ADIEU'.
(Other definitions for adieu that I've seen before include "Goodbye, mon ami!" , "Fare thee well!" , "A valediction" , "Au revoir (to a kind friend?)" , "Farewel" .)